Portable electronic meters are typically used to test vehicle engine parameters. An electronic meter may comprise a simple hand held tachometer with a display for showing the engine speed, or may comprise a more elaborate multimeter or computer console, capable of displaying additional engine parameters such as dwell angle, resistance, and voltage levels. A typical digital multimeter, for example, is found in the Model 2880 multimeter produced by KAL Equip Company of Cleveland, Ohio. A typical digital multimeter, including the Model 2880, includes a series of input jacks, a multiple position rotary switch, and a display. Additionally, multimeters can have power leads adapted to supply power to the meter from the vehicle's battery.
When the engine speed is being tested with a multimeter, a pickup for the multimeter is positioned around a spark plug lead. The pickup is adapted to inductively measure the current flowing through the lead. The changing current in the lead, for example when the spark plug is ignited, produces a changing magnetic field. The changing magnetic field in turn produces a changing current, manifested as a current "spike", in the inductive pickup. The inductive pickup applies the current spike to the multimeter through one of the input jacks in the meter. If the current spike is above a selected threshold value, the meter applies a selected duration voltage pulse to a conventional RC integrating circuit. The meter then scales, measures and displays the voltage across the integrating circuit as a value of engine speed (in revolutions per minute or "RPM").
The rotary switch of the multimeter is adapted to be selectively positioned. When engine speed is being tested, the rotary switch is typically set to a "tachometer" position. The rotary switch is in registering relationship with electrical components in the multimeter to properly scale and display the engine parameters on the meter.
A multimeter is typically designed to measure the engine parameters of an internal combustion engine. An internal combustion engine comprises either a two or four stroke engine. A two stroke engine is typically found in smaller engines, such as outboard motors, chain saws and lawn mowers, while a four stroke engine is typically found in larger engines, such as automobiles and trucks. Moreover, a four stroke engine can either have a conventional distributor-type ignition system, or can have a newer distributorless ignition system ("DIS"), which is typically found in late model cars and is electronically controlled by an on-board computer.
A two stroke engine requires two strokes of the piston within the cylinder to complete the operating cycle of the engine. For example, a two stroke engine typically has intake and exhaust port openings on the side of the cylinder in the crankcase. The piston alternately opens and closes the openings as the piston moves within the cylinder.
In a typical two stroke engine, the piston initially moves towards the upper end of the cylinder and creates a pressure drop in the crankcase. The pressure drop in the crankcase creates a flow of fuel and air from the carburetor into the crankcase cavity through a one-way reed valve. As the piston begins to move toward the bottom of the cylinder, during the "power" stroke, the valve closes, thereby trapping and compressing the air-fuel mixture in the crankcase. As the piston continues to move to the bottom of the cylinder, a fuel intake bypass port is uncovered in the cylinder, and the compressed fuel-air mixture flows from the crankcase into the combustion chamber of the cylinder.
As the piston again starts its upward, "compression" stroke, the piston covers the bypass port and compresses the fuel-air mixture in the compression chamber. When the piston nears the top of the compression stroke, the compressed mixture is ignited by a spark across the gap of the spark plug. The spark is caused by an ignition coil which creates a voltage drop across the spark plug gap. The rapid burning of the fuel forces the piston towards the rear of the cylinder on the power stroke. When the piston reaches the bottom of the power stroke, an exhaust port opening is uncovered, slightly ahead of the fuel intake port opening. The exhaust gas, which is still under a slight pressure, escapes through the exhaust port opening, and is assisted by the incoming fuel-air mixture as the fuel intake port opening is uncovered by the piston.
Alternatively, both a conventional and DIS four stroke engine require four strokes of the piston to complete the operating cycle of the engine. A four stroke engine has (1) an intake stroke, (2) a compression stroke, (3) a power stroke, and (4) an exhaust stroke.
On the intake stroke of both the conventional and DIS four stroke engine, the piston moves toward the bottom of the cylinder and creates a vacuum above it in the head of the cylinder. A camshaft mechanically coupled to the crankshaft causes an intake valve in the head of the cylinder to open, and an exhaust valve to close. The intake valve delivers a fuel-air mixture to the cylinder. When the piston begins to move upward in the cylinder during the compression stroke, the intake valve closes and the air-fuel mixture is compressed. When the piston nears the top of the compression stroke, the spark plug fires and ignites the fuel mixture. The rapid burning of the fuel forces the piston downward during the power stroke. At the bottom of the power stroke, the exhaust port opens and the exhaust gas flows out the port, assisted by the upwardly moving piston during the exhaust stroke. Additionally, in a DIS four stroke engine, the spark plug fires a "waste spark" during the exhaust stroke, the relevance of which will be described herein in more detail.
The crankshaft, which is mechanically attached to the end of the piston, rotates once each time the piston reciprocates within the cylinder. The crankshaft is adapted to be mechanically coupled to the vehicle axle to provide power for moving the vehicle.
As described above, the spark plugs in a two stroke engine and a conventional four stroke engine fire only during the power stroke. Therefore, since the piston in the two stroke engine has only two strokes per cycle (a power stroke and a compression stroke), the spark plugs in the two stroke engine fire each time the piston moves to the upward end of the cylinder. In a conventional four stroke engine, the engine has four strokes per cycle (an intake stroke, a compression stroke, a power stroke and an exhaust stroke), therefore, the spark plugs in a conventional four stroke engine fire once every other time the piston moves to the upper end of the cylinder. However, in a DIS four stroke engine, although the engine has the same general cylinder structure as a conventional four stroke engine, the spark plugs fire each time the piston moves to the upper end of the cylinder, which emulates more the engine diagnostics of the two stroke engine rather than the engine diagnostics of a conventional four stroke engine, as will be described herein in more detail.
A DIS four stroke engine has a series of double ended coils which each fire two spark plugs, each in their own separate cylinders, simultaneously. Each coil is coupled through an ignition module to a timing circuit, which is included within an on-board computer. The first spark plug on the coil fires normally in a first cylinder that is on a compression stroke and ignites the air/fuel mixture, while the second spark plug fires a "waste spark" in a second cylinder that is on an exhaust stroke, so named because the spark does not ignite an air/fuel mixture. Consequently, the DIS four stroke engine emulates the two stroke engine because the spark plugs fire each time the piston moves to the forward end of the cylinder.
Moreover, the ignition system of the DIS four stroke engine is more efficient than a conventional four stroke engine. The amplitude and duration of the current in the primary windings of the ignition coils are closely controlled in the DIS four stroke engines. Accordingly, after the spark plugs are ignited, the current flowing through the spark plug leads in the DIS four stroke engine is reduced, and there is a corresponding reduction in "noise" in the spark plug leads caused by other cylinders.
Conventional electronic meters are designed to measure the engine speed for either a two stroke or conventional four stroke engine. For example, the pickup for a two stroke electronic meter is placed around one of the spark plug leads on the two stroke engine and an appropriate value of engine speed is displayed on the meter. However, if the pickup for a meter designed for a two stroke engine is placed around the spark plug lead for a four stroke engine, the meter will display an engine speed which is half the appropriate value, since the spark plugs on a four stroke engine are being fired at half the rate of the spark plugs on a two stroke engine. Similarly, an electronic meter designed for a four stroke engine will display twice the value of engine speed for a two stroke engine.
Moreover, if a DIS four stroke engine is measured using a conventional four stroke electronic meter, the meter will display twice the appropriate value of engine speed, since, similar to the two stroke engine, the spark plugs in the DIS four stroke engine are firing twice as rapidly as a conventional four stroke engine. For service personnel testing engine diagnostics on cars and trucks, and in particular the engine speed, it is therefore necessary to use either a meter designed for a two stroke engine on the DIS four stroke engine, or to manually divide in half the constantly fluctuating engine speed displayed on a conventional four stroke electronic meter, to arrive at the proper engine speed value.
It is known in the art to provide an electronic meter with a switch that allows the meter to properly scale and display the engine speed of both a conventional four stroke or two stroke engine. Such a meter is manufactured by Actron, Model No. ET915. The meter essentially divides the engine speed in half before displaying the value on the meter. However, since the current levels in the spark plug leads for a DIS four stroke engines are less than the current levels for a conventional four stroke engine, electronic meters designed for a conventional four stroke engine can in some instances provide erroneous readings as to engine speed for a DIS four stroke engine. The error results primarily because the threshold current levels for triggering a conventional four stroke meter can be higher than the maximum current level in a DIS four stroke engine.